1. Field of the Invention
The present invention relates to an optical scanning of the type scanning a plurality of image carriers with a plurality of laser beams issuing from a plurality of light sources, and a printer, copier, facsimile apparatus or similar image forming apparatus using the same.
2. Description of the Background Art
A recent achievement in the imaging art is a color laser printer or similar image forming apparatus using a plurality of laser beams for scanning a plurality of image carriers or photoconductive elements. In this type of image forming apparatus, each laser beam issuing from a particular light source is focused on one of the image carriers to form a latent image. The latent image is developed by a developer of particular color to thereby produce a corresponding toner image. Such toner images of different colors are sequentially transferred to a single sheet or recording medium one above the other, completing a full-color image. A problem with this type of image forming apparatus is that a plurality of scanning means necessary for dealing with the laser beams increase the number of parts and obstruct the free layout of parts and therefore make the apparatus bulky, compared to a monochromatic image forming apparatus.
To meet the increasing demand for a color laser printer as small as and as productive as a monochromatic laser printer, there has recently been implemented a tandem image forming apparatus including a plurality of optics arranged in a single housing and a plurality of image carriers arranged side by side. Each optics includes a light source for emitting a laser beam for scanning a particular image carrier. An optical scanning device included in this type of image forming apparatus includes a rotatable deflector made up of two polygonal mirrors stacked on each other. However, because the two polygonal mirrors have the same angular phase, lights propagated through two optical paths are incident to a single synchronization sensor at the same time, making it difficult to produce synchronizing signals independent of each other, as will be described more specifically later.
Another problem with the above scanning device is that the synchronization image height of the individual laser beam is not always constant. It is therefore necessary to provide a difference between image heights great enough to insure the separation of the resulting synchronizing signals. Such a difference, however, is not practicable without increasing the effective scanning range and therefore the sizes of lenses and other optical devices. Large-size optical devices make the scanning device and therefore the entire image forming apparatus bulky while increasing production cost.
To further enhance the productivity of the image forming apparatus using the above scanning device, the deflector must be rotated at high speed, e.g., about 30,000 rpm (revolutions per minute). This, however, makes the separation of synchronizing signals more difficult and reduces the service life of a motor used to drive the deflector while aggravating noise during operation.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 11-142771, 2000-280523 and 2001-108923.